Compile fix for gcc 4.8.3

Removed compile breakting redefinition of UA_Subscription

src/ua_session.h

@@ -17,7 +17,6 @@ struct ContinuationPointEntry {
 };
 
 struct UA_Subscription;
-typedef struct UA_Subscription UA_Subscription;
 
 struct UA_Session {
     UA_ApplicationDescription clientDescription;